1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus which converts a light signal into an electric signal corresponding to intensity of the light signal, and to a photoelectric conversion system using the photoelectric conversion apparatus. More particularly, the invention relates to a photoelectric conversion apparatus which enables to easily convert a light signal into an electric signal having a desired magnitude to output the electric signal, and also relates to a photoelectric conversion system using the photoelectric conversion apparatus.
2. Description of the Related Art
In optical communication systems, measuring apparatuses for optical communication, optical wirings, optical computers, and so on, it is frequent that a photoelectric conversion apparatus converts light signals into electric signals, and that electric processing is performed on the electric signals in a subsequent-stage circuit. Further, in the case of using multi-input signals (for example, in the case of separating components of a multiplexed light signal, such as a wavelength division multiplexed light signal and a time division multiplexed light signal), a photoelectric conversion apparatus is provided in such a way as to be associated with each of the light signals.
FIG. 4 is a view illustrating the configuration of a portion for separating multiplexed light signals and for converting the light signals into electric signals in an optical communication system (see, for example, the following document (1)). An arrayed waveguide grating 10 (hereunder abbreviated as AWG) shown in FIG. 4, to which multi-wavelength light signals transmitted in the optical communication signals (having wavelengths λ1, λ2, λ3, . . . ) are inputted, demultiplexes (that is, performs wavelength-based separation of) the inputted light signals into light signals of wavelengths λ1, λ2, λ3, . . . and outputs the demultiplexed light signals (see, for instance, the following document (2)).
The light signals respectively having the wavelengths are inputted from the AWG 10 to delay fibers DF1 to DF3. For example, the light signal having the wavelength λ1 is inputted to the delay fiber DF1. The light signal having the wavelength λ2 is inputted to the delay fiber DF2. The light signal having the wavelength λ3 is inputted to the delay fiber DF3. Then, the delay fibers DF1 to DF3 delay the light signals by a predetermined time, and outputs the delayed light signals. Variable optical attenuators AT1 to AT3 are provided in such a way as to be associated with the delay fibers DF1 to DF3, respectively. The light signals are inputted from the delay fibers DF1 to DF3 to the variable optical attenuators AT1 to AT3, respectively. Each of the variable optical attenuators AT1 to AT3 attenuates the intensity of the light signal and outputs the attenuated light signal.
Photoelectric conversion apparatuses 20a to 20c are provided in such a manner as to be associated with the variable optical attenuators AT1 to AT3, respectively. Light signals are inputted from the variable optical attenuators AT1 to AT3 to the photoelectric conversion apparatuses 20a to 20c, which convert these light signals into electric signals and subsequently output the electric signals. Further, each of the photoelectric conversion apparatuses 20a to 20c has a photodiode 21, a resistor R, an operational amplifier 22, an output terminal 23, and a bias circuit BC. The photodiode 21 is connected to the bias circuit BC at the cathode thereof, to which a bias voltage is applied. The operational amplifier 22 is connected to the ground GND at the non-inverting input terminal thereof, and also connected to the anode of the photodiode 21 at an inverting input terminal thereof. The output terminal 23 of each of the photoelectric conversion apparatuses 20a to 20c is connected to that of the operational amplifier 22. Further, the resistor R is connected to the non-inverting input terminal and the output terminal of the operational amplifier 22 to thereby constitute a negative feedback loop.
Incidentally, FIG. 4 shows only three of each of the three kinds of constituents, that is, the delay fibers DF1 to DF3, the variable optical attenuators AT1 to AT3, and the photoelectric conversion apparatuses 20a to 20c provided in the portion. However, needless to say, a necessary number of each of such kinds of constituents may be provided therein. Additionally, the configuration of each of the photoelectric conversion apparatuses 20b and 20c is similar to that of the photoelectric conversion apparatus 20a. Thus, only the photodiode 21 is shown in this figure.
An operation of such a portion is described hereinbelow.
Wavelength division multiplexed light signals are inputted to the AWG 10. Then, the AWG 10 demultiplexes the light signals and outputs the demultiplexed light signals to the delay fibers DF1 to DF3. Subsequently, the delay fibers DF1 to DF3 delay the light signals, which are outputted from the AWG 10, and output the delayed light signals to the variable optical attenuators AT1 to AT3, respectively.
Then, the variable optical attenuators AT1 to AT3 attenuate the light signals. Incidentally, an attenuation amount is adjusted by taking differences in intensity among the light signals and those in voltage level, which are caused when the conversions are performed in the photoelectric conversion apparatuses 20a to 20c, into consideration.
Subsequently, the light signals attenuated by the variable optical attenuators AT1 to AT3 are inputted to the photodiodes 21 of the photoelectric conversion apparatuses 20a to 20c, respectively. Then, the photodiodes 21 outputs electric signals having voltage levels, the value of each of which corresponds to the intensity of the associated light signal, to a subsequent-stage electric circuit (not shown).
Next, an example of demultiplexing the light signals by using optical couplers and wavelength filters without using the AWG 10 is described hereinbelow. FIG. 5 is a view illustrating another configuration of a portion for separating components of a multiplexed light signal and converting the light signal into electric signals. In this figure, constituents, which are the same as those shown in FIG. 4, are designated by the same reference characters denoting the same constituents shown in FIG. 4. Thus, the description of such constituents is omitted herein.
As shown in FIG. 5, the optical couplers CP1 to CP3 and the variable wavelength filters FL1 to FL3 are provided in the portion, instead of the AWG 10. The optical coupler CP1, to which the wavelength division multiplexed light signal is inputted, into two light signals. Each of the optical couplers CP2 and CP3 braches the light signal, which is inputted thereto from one of output terminals of a preceding-stage optical coupler CP1 or CP2, into two light signals. The branching ratio of the intensity of each of the optical couplers is 1:1.
The light signals are inputted from other output terminals of the optical couplers CP1 to CP3 to the variable wavelength filters FL1 to FL3, respectively. The variable wavelength filters FL1 to FL3 transmit only light signals of predetermined wavelengths and output the transmitted light signals to the delay fibers DF1 to DF3, respectively.
An operation of such an apparatus is described hereinbelow.
Wavelength division multiplexed light signals are inputted to the optical coupler CP1. Then, the optical coupler CP1 branches the light signal into two light signals and outputs the two light signals to the variable wavelength filter FL1 and the optical coupler CP2. Similarly, the optical coupler CP2 branches a light signal, which is inputted thereto, into two light signals and outputs these two light signals to the subsequent-stage optical coupler CP3 and the variable wavelength filter FL2. Furthermore, similarly, the optical coupler CP3 branches the light signal inputted thereto into two light signals and outputs these two light signals to a subsequent-stage optical coupler (not shown) and to the variable wavelength filter FL3.
Further, the variable wavelength filters FL1 to FL3 transmit only the light signals of desired wavelengths and output the transmitted light signals to the delay fibers DF1 to DF3. For example, the variable wavelength filter FL1 transmits only the light signal of the wavelength λ1. The variable wavelength filter FL2 transmits only the light signal of the wavelength λ2. The variable wavelength filter FL3 transmits only the light signal of the wavelength λ3.
Additionally, the operations of delaying the light signals by using the delay fibers DF1 to DF3, and of attenuating the light signal by using the variable optical attenuators AT1 to AT3, and of converting the light signals into electric signals by using the photoelectric conversion apparatuses 20a to 20c are similar to those of the apparatus shown in FIG. 4. Thus, the description of such operations is omitted herein.
The following documents are referred to as related art.
(1) JP-A-2000-244458 (Paragraph Nos. 0020 to 0024 and FIGS. 1 and 2)
(2) H. Takahashi, I. Nishi, and Y. Hibino: “10 GHz SPACING OPTICAL FREQUENCY DIVISION MULTIPLEXER BASED ON ARRAYED-WAVEGUIDE GRATING”, ELECTRONICS LETTERS, Institution of Electrical Engineers, February 1992, Vol. 28, No. 4, pp. 380–382.
Thus, the demultiplexed light signals are converted by the photoelectric conversion apparatuses 20a to 20c into electric signals. However, it is necessary for processing the converted electric signals that the signal levels (for instance, a high or low voltage level) of the electric signals have the same value. Needless to say, the same goes for the case of separating components of the time division multiplexed light signal.
Thus, in a case where such signal levels are adjusted by adjusting the levels of the light signals, the variable optical attenuators AT1 to AT3 adjust those of the light signals. Alternatively, in a case where such signal levels are adjusted by converting the light signals into electric signals, an amplification circuit (not shown) is provided in a subsequent stage of each of the photoelectric conversion apparatuses 20a to 20c. Such signal levels are adjusted by this amplification circuit.
However, when the photoelectric conversion apparatuses 20a to 20c convert the light signals into the electric signals, the corresponding relation between the intensity of the light signal and the signal level of the electric signal is fixed. Thus, when all the multiplexed light signals vary or when only the light signal of a specific wavelength varies, it is necessary to adjust the attenuation amount of each of the variable optical attenuators AT1 to AT3 and the gain of the amplification circuit (not shown) according to the values of the intensity of the light signal, which is not demultiplexed, and of the intensity of the demultiplexed signal, respectively. Consequently, such adjustment is complicated and troublesome.
Furthermore, in the actual system shown in FIGS. 4 and 5, there has been a variation in the insertion loss of the individual optical components, such as the AWG 10, the optical couplers CP1 to CP3, the variable wavelength filters FL1 to FL3, the delay fibers DF1 to DF3, the variable optical attenuators AT1 to AT3, and the photodiodes 21. Thus, the adjustment cannot be achieved simply by changing the resistance value of the resistor R of each of the photoelectric conversion apparatuses. There has been necessity for fine adjustment of the attenuation amount and the gain. Thus, such adjustment is complicated and troublesome.